The present invention relates to a rolling bearing, particularly, to a long-lived rolling bearing that is improved in grinding efficiency and that is suitable for use on transmissions and engines in automobiles, agricultural machines, construction machines, iron and steel machines, etc, and further relates to a heat treatment method for hardening a surface to produce the long-lived rolling bearing.
Rolling bearings are conventionally produced by a processes that typically provides rolling a material of carburizing steel, which is then forged under hot, warm or cold conditions and cut by turning. In order to improve the life of rolling bearings, various surface hardening treatments are performed, such as carbonitriding in the temperature range from 650 to 900.degree. C., followed by oil quenching, or carburizing in the temperature range from 900 to 950.degree. C., followed by carbonitriding in the temperature range from 800 to 860.degree. C., which in turn is followed by oil quenching. These surface hardening treatments contribute to a marked improvement in the temper resistance of the rolling bearing, whereby its resistance to wear and corrosion is sufficiently increased to prolong its life.
Rolling bearings are used in increasingly hostile environments and the demand for extending their life has accordingly become rigorous.
Under the circumstances, Examined Japanese Patent Publication No. Sho. 62-24499 and Unexamined Japanese Patent Publication No. Hei. 2-34766 proposed that a heat treatment such as carburization be performed in such a way as to precipitate spheroidal cutting carbides on the surface of a low to medium carbon, low alloy steel, thereby improving the hardness of the steel surface (the surface of outer and inner races, as well as rolling elements) while extending the life of the rolling bearing.
Rolling bearings are often used under dirty lubricating conditions in the presence of foreign matter. Unexamined Japanese Patent Publication No. Sho. 64-55423 proposes that the content of carbon, the amount of residual austenite (.gamma..sub.R vol %) and the content of carbonitrides in the rolling surface layer of a bearing be adjusted to appropriate values, whereby the stress concentration at the edges of dents formed by the foreign matter is sufficiently reduced to suppress the occurrence of cracking while extending the rolling life of the bearing.
The conventional method described in Examined Japanese Patent Publication No. Sho. 62-24499 and Unexamined Japanese Patent Publication No. Hei. 2-34766 which is directed to the enhancing of the surface hardness of the races and rolling elements is effective in lessening the severity of dents formed by foreign matter. On the other hand, the toughness of the races and rolling elements decreases and cracks propagate from the areas of damage caused by the foreign matter in the lubricating oil; such cracks serve as the start point for early flaking which makes it impossible to achieve satisfactory extension in the life of the bearing.
The conventional method described in Unexamined Japanese Patent Publication No. Sho. 64-55423 is effective in extending the life of a rolling bearing under dirty lubricating conditions due to the presence of an appropriate amount of residual austenite. On the other hand, the residual austenite lowers the surface hardness of the bearing and deteriorates its fatigue resistance. Thus, there still exists the need to establish a more appropriate relationship between the amount of residual austenite (.gamma..sub.R vol %) and the surface hardness (Hv).
Under the circumstances, the present applicant has previously discovered an optimal relationship between the amount of residual austenite (.gamma..sub.R vol) in the rolling surface layer of a rolling bearing and its surface hardness (Hv). Further, the present applicant has proposed a long-lived rolling bearing in which the average particle size of carbides and carbonitrides in the rolling surface layer were adjusted to optimal values, thereby solving the problem of reduced surface hardness due to the presence of residual austenite.
Unexamined Japanese Patent Publication No. Hei. 3-24258 proposed a method providing nitriding a carburizing steel part in the temperature range from 750 to 800.degree. C. and then carburizing the same at a temperature not lower than 900.degree. C. or carbonitriding it at a temperature not lower than 800.degree. C. The nitriding treatment drives nitrogen into the surface layer, thereby increasing the surface nitrogen concentration and the subsequent carburizing treatment allows the nitrogen to diffuse so that it penetrates deep into the surface layer together with carbon, thereby providing a long-lived rolling bearing.
The rolling bearing just described above has been carbonitrided at temperatures in the range from about 650 to about 900.degree. C., which are conventionally referred to as the "common treatment temperatures". However, with such rolling bearing that has been subjected to carbonitriding at the common treatment temperatures, a more-than-necessary large amount of nitrogen is contained in the heat-treated surface layer, so carbonitrides are prone to be precipitated in large amounts and, what is more, such excess nitrogen will contribute to unduly improved resistance to tempering and war. As another problem, residual austenite which is present in a more-than-necessary large amount lowers the grinding machinability of bearings. Thus, the method proposed in Unexamined Japanese Patent Publication No. Hei. 3-24258 has not given due consideration to the grinding machinability problem.
A further problem with the carbonitriding and nitriding treatments under discussion is that since they are performed at lower temperatures than ordinary carburizing treatments, an unduly long time is taken to produce a hardened layer of the same nature as the one that is produced by carburization; as a result, the productivity drops but the initial cost increases.
It should also be mentioned that the carbonitriding treatment performed at the aforementioned temperature takes so much time that carbonitrides tend to become coarse in structure in the flow direction of material and serve as the start point for the propagation of cracks, thereby lowering the fatigue life of the final product.